I want to restart Rainlandar from another program and I thought there might be a command line switch to do that.

What I'm doing is sync'ing a private database of events with Rainlandar. What I do is to make a local copy of the Default.ics file, merge in my events, then replace the Default.ics file with my updated .ics file. In order for Rainlandar to see the changed Default.ics file, I need to kill it, then restart it. I have been doing that manually, but now I want to do it programatically.

From the help file there's no option to refresh Rainlendar2 by command line.
But if you have some knowledge of programming, then you can create a lua script using the Rainlendar_Refresh() function and put it on the app path. Then, there is a command line to execute this function:-e
--execute
Name of a function
Executes the given function in a lua script. The script must have been loaded by Rainlendar (i.e. it must be used by the current skin or the file needs to be located in the Rainlendar's scripts folder).

EDIT: the script file rainlendar2/scripts/hotkeys.lua already has this function. It's called Hotkey_Refresh(). Maybe you could use that.